The filtration of particulate materials from fluids has been conducted for years relying mainly on sieving mechanisms, where particles are predominantly removed based on size. Sieving by itself in many cases has not been satisfactory for liquid filtration as it provides poor or no retention for particles smaller than the pore size of the filter, as well as low flow rates requiring either large pressure gradients or large filter areas to attain a reasonable flow. However, particles can be removed from liquids relying on attractive particle-surface interactions, a practice which has long been recognized in gas filtration, where filtration mechanisms other than sieving dominate in most applications and provide enhanced filter performance. See, Particle Capture Mechanisms in Gases and Liquids: An Analysis of Operative Mechanisms, by Grant, et. al., 1988 Proceedings of the Institute of Environmental Sciences.
Particle-surface interactions in liquids are usually governed by electrostatic interactions and Van der Waals Forces assuming that no other highly specific interaction potentials are present (as e.g. molecular recognition, chelating functionalities etc.). Van der Waals forces are omnipresent attractive short ranged forces acting between two materials of any kind and will thus always represent attractive interactions. Electrostatic interactions on the other hand, will be attractive, repulsive or non-existent dependent on the sign of the electrostatic potential of the two materials in question. It is common knowledge, that like-charged materials will exhibit repulsive electrostatic interactions between each other, whereas counter-charged material will exhibit attractive electrostatic interactions between one another. This knowledge has been exploited in the filtration of liquids, where the filter material exhibiting electrostatic potential is capable of attracting and successfully retaining countercharged particles. The obvious limitation of these charged filters are the repulsive electrostatic interactions of the filter material with like-charged particles and the resulting poor retention for the same.
The basis of this invention is the realization that when one of the material surfaces in question is neutral in the liquid of use (here the filter material), no adverse electrostatic interactions will be present between the filter material and the particle material and attractive Van der Waals forces will govern the interaction between the filter material and particle material leading to the retention of particles of any charge characteristic, positive, negative, or neutral. Therefore, in providing a filter material that exhibits no, or essentially no, electrostatic potential in the liquid to be filtered (i.e. it is a neutral surface), particles of any electrostatic character will be attracted by the filter material leading to enhanced retention and filtration beyond that currently obtained with conventional sieving only filters.
The electrostatic potential of materials in liquids are governed by two main phenomena, (i) the dissociation/association of functional groups (acids or bases) leading to a charge, or (ii) the adsorption of (charged) ionic species from the liquid. For example, in aqueous media, materials without any functional groups at the material surface usually exhibit a negative electrostatic potential in basic or neutral solutions (pH>5-7) and positive electrostatic potential in acidic solutions (pH<5-7) due to the adsorption of hydroxyl (negatively charged: OH−) or hydronium ions (positively charged: H+). Independent of the character of the liquid (aqueous or non-aqueous) charged ionic species present in the filtration liquid might also adsorb to the material surface and therefore alter its electrostatic potential.
It is therefore important to realize, that a material attains a different electrostatic potential in dependence on the properties of the liquid it is immersed in. The electrostatic potential of a material in a liquid will depend on properties of the liquid such as (i) its proton donor/acceptor capability, (ii) its dielectric constant, (iii) the concentration and kind of ionic species present. Nevertheless, through the use of appropriate materials or surface modifications of the same, one can adjust the electrostatic potential of a material in a given liquid to be essentially zero thereby creating an essentially neutral surface.
Based on this finding, this invention teaches that a filter material can be selectively adjusted for a given fluid in a given pH range to attract particles of any electrostatic character and can therefore effectively retain particles smaller than the pore size of the filter based on attractive interactions between the particle and the filter material in any given liquid.